Is your voltage control in your BIOS set to [AUTO] or [NORMAL]? If so, that could be the reason your voltages are fluctuating. As specified by Intel, the factory-warranted voltage range for a E6400 is 0.850v - 1.325v. Thus, your motherboard is probably adjusting the voltage as necessary. More voltage is more heat, so why make the processor hotter than it needs to be?
Just set the voltage manually to [1.325v], overclock until you're happy, and then lower the voltage as much as you can. Remember to try and keep the temperature of the cores under 50c (65c in Core Temp). Additionally, when overclocking, make small jumps in frequency changes; try a 200MHz overclock and test it before going any further.
EDIT: And, by the way, the +5, +12, -12, -5, etc rails are almost never read right in Speedfan. I only have a 500w power supply in my E6300 (3.1GHz) paired with power-hungry components, so you'll be fine.